Prostate cancer (PCa) is the most commonly diagnosed non-cutaneous malignancy and the second leading cause of cancer-related deaths in US males. PCa at all stages is dependent on the activity of the androgen receptor (AR), and consequently targeting this pathway has been a focus for therapeutic intervention. Additionally, Myc is one of a few bonafide oncogenes imperative to PCa progression and aggressiveness, which is often deregulated through the disease course and represents a potential, but currently undruggable, target for PCa. Radiation therapy, often combined with androgen deprivation therapy (ADT), is standard of care for patients presenting with organ confined or locally advanced disease, and has recently been approved for treatment of metastatic disease. Deregulation of AR and Myc are both implicated in decreased sensitivity to radiotherapy. Therefore, novel therapies that inhibit AR and/or Myc signaling could effectively radiosensitize PCa cells, thereby improving disease management. It is well established that sphingolipid metabolism plays key roles in tumor biology. In particular, sphingosine kinases (SK1 and SK2) are a potential site for manipulation of the ceramide / sphingosine 1- phosphate rheostat that regulates the balance between tumor cell proliferation and apoptosis, as well as tumor sensitivity to radiation. We and others have demonstrated that SKs are frequently overexpressed in many human cancers, including PCa, and that inhibition of SK activity has anti-proliferative effects on tumor cells. Apogee Biotechnology Corporation has identified orally-available SK inhibitors with activity in vitro and in vivo. The lead SK2 inhibitor, designated as ABC294640, has antitumor and anti-inflammatory activities in several in vivo models, while exhibiting minimal toxicity. ABC294640 has recently completed phase I clinical testing in patients with advanced solid tumors, but has not been previously assessed in PCa patients or preclinical models. We have now found that ABC294640 effectively decreases AR and Myc expression and activity, attenuates PCa cell growth, and inhibits in vivo tumor growth. Therefore, we hypothesize that ABC294640 can radiosensitize PCa in vitro and in vivo and significantly diminish tumor growth. To support expanding clinical trials of ABC294640 into PCa patients, we will conduct studies to determine the therapeutic efficacy of ABC294640 in in vitro and in vivo models of PCa in combination with a relevant radiotherapy regimen. The following Specific Aims will be addressed in this phase 1 SBIR project: 1. To analyze the in vitro capability of ABC294640 to sensitize PCa cells to radiation therapy; 2. To determine the mechanism for ABC294640-mediated radiosensitization; and 3. To evaluate the ability of ABC294640 to modify radiation therapy for PCa in vivo. Clear mechanistic rationale and extensive Preliminary Studies support the hypothesis that ABC294640 will provide a new effective therapy for PCa that uniquely targets multiple pathways driving disease aggressiveness, progression, and resistance to therapy. Successful completion of this project will leverage our clinical experience with ABC294640 by providing justification to the FDA to expand our clinical trials into PCa, a disease with a high unmet clinical need.